The present invention relates to an apparatus for uniting at least two concentrically extruded tubular films of thermoplastic synthetic resin wherein at least one of the films is heated to a temperature slightly below the melting point of the resin from which it is formed.
In order to obtain an end product of high tear strength, the extruded tubular films are stretched in such an apparatus in different directions. The outermost tubular film is stretched longitudinally without changing its diameter and each of the inner extruded tubular films, which have a smaller diameter than the outermost film, are stretched in a direction transverse to their longitudinal direction by a heated mandrel located within the innermost film and having a maximum diameter substantially equal to the diameter of the outermost film.